The present invention relates to a pump. More particularly this invention concerns an adjustment arrangement for a pump wherein the rotor carries a plurality of radially movable displacement elements whose strokes can be adjusted by a ring surrounding the rotor which is in turn radially positioned by a pair of diametrically opposite pistons.
A radial-piston pump is known whose rotor is surrounded by an adjustment ring against one radial side of which bears a relatively large-diameter piston and against its other side bears a relatively small-diameter piston. The pump output, that is fluid pressurized by the radially-displaceable pistons in the rotor, is fed directly to the small-diameter piston which bears on its radially inner end against the adjustment ring.
In such an arrangement it is customary to form a passage extending radially of the rotor through the small-diameter piston and terminating at the inner end of this small piston at a so-called pressure-relief or depressurizing formation. Such a formation serves to reduce the surface contact between the adjustment ring and the small piston as well as to reduce the force necessary for adjustment of this ring. Nonetheless such a depressurizing has the effect that in the central position of the ring, that is when it is coaxial with the rotor, and when the output pressure of the pump is very low, the friction between the adjustment ring and the small piston is also very low. Thus, the rotor frequently rotationally entrains the adjustment ring due to the friction between the various pumping pistons in the rotor and this ring so that very quickly the ring is brought up to the same rotational speed as the rotor. Such entrainment of the adjustment ring has a gyro effect so that much more force is needed than normally to radially displace it.